Set Freedesktop naming as default for a smoother transition

Rewritten the options extension from scratch. Moved it to a separate listing;
Deprecated Freedesktop version, the equivalent functionality is now provided by the new options extension;
Totally new icon system based on CSS variables. Icon themes can now be easily changed via options extension;
Updated addon support styles;
Dropped ChatZilla and Inspector support, because they were of a very low quality;
Lots of small tweaks everywhere.

Rebased on the latest Toolkit and SeaMonkey Default code, fixing many long-standing bugs;
Formatting toolbar in Composer and Mail Compose windows now uses small icons;
Redesigned Sidebar tabs;

Options addon rewrite and changeable icon themes have been postponed to 0.4. That version will also deprecate GNOMErunner/GTK Revived Freedesktop theme in favor of the regular GNOMErunner/GTK Revived, which will include an option for choosing Freedesktop icon naming instead of stock GTK one. Freedesktop theme will be removed in 0.4.1.

Bumped SM version

SeaMonkey 2.29 compatibility;
SeaTabX2 support.

SeaMonkey 2.24, 2.25, 2.26 compatibility;
Improved sidebar borders;
Fixed compatibility with new Lightning versions;
Fixed SeaFox addon manage categories;
Many small fixes.

• Added SeaFox 0.2 support.

0.3:

• First release of separate Freedesktop variant, before it was an option in configuration addon.